Drilling advances

	Vol. 227 No. 8
LES SKINNER, PE CONTRIBUTING EDITOR

Lost circulation. Lost circulation is the loss of returned mud from the annulus of a conventionally circulated well. From a hydraulic standpoint, it means that the mud column equivalent circulation density (ECD) exceeds the ability of the formation to resist injection. The reasons are numerous: naturally fractured formations or vugular porosity in which the formation pore pressure is the only resistance to mud loss (i.e., formation pore pressure equals “fracture” pressure), high-porosity/ high-permeability formations, leaks to upper intervals of various kinds (behind pipe, at a bad shoe and up a fault plane), induced fractures (stimulations or waterflooding at high pressures) and other types of induced interwell communications (gas zones, fluidized beds, shale planes, etc.).

In his book Field Guide to Lost Circulation, author Bill Murchison refers to lost circulation as “...the number one problem in drilling...” Bill could not be more correct. This problem has plagued drillers since the first brine well in China was sunk using spring poles. My grandfather, an old water well driller in the early 1900s, told me horror stories of “the bottom falling out” while they were circulating gelled muds to condition the hole. Often, they would lose the entire well as the walls caved in. They would move over a few feet and drill another well, only to lose that one, too. Needless to say, Daddy Bun hated lost circulation.

Early mud systems were fairly primitive from today’s viewpoint. Accounts of running cattle through a pit in a rice field to stir up “gumbo” for pumping down the hole are found in early records (actually, this is not a bad mixing system from a rig horsepower standpoint). Chances are the mud produced by this method contained rice hulls, whole rice grains, chafe, grass, weeds and other unmentionable organic material deposited by the cattle (who probably didn’t approve of being abused as mud mixers – after all, there are roughnecks for this type work). The mud successfully “plastered” the borehole walls allowing drilling to continue to the delight of the rig crews and working interest owners.

Once mud systems became more sophisticated, the need for bridging materials became critical. The materials mixed in early water-based mud systems is almost limitless. Just about anything that could be pumped downhole to “plaster” the wall was used including various fibers, cottonseed, dried cellulose, kenaf, rubber chips, nut hulls, oyster shells, leather strips, rice, beans, okra and tumbleweeds. Of this group, I like tumbleweeds best. Max Hall and I pumped thousands of bushels of these noxious weeds down wells in West Texas years ago, along with tons of red blow sand, to cure lost circulation. Don’t laugh; it worked.

New lost circulation products have been introduced over the years and used in the industry with varying success. Nut hulls have remained a mainstay. These are ground to grades described only as coarse, medium or fine (nobody has yet told me what separates the categories), and they include shells from walnuts, cottonseed, Brazil nuts, filberts, pecans, peanuts and almonds. Now we are discovering that some of them are useful for other purposes. Extended reach drilling (ERD) takes advantage of the “birds nest” created by walnut hulls to improve hole cleaning. Coarse walnut hulls and shredded almond shells improve borehole lubricity in ERD operations in the North Sea and the Caspian Sea. Peanut hulls mixed with a polymer have been so successful in stopping lost circulation that they have become an exclusive product line.

Cellulose products include shredded paper, various plant fibers, chopped cotton cloth, wool, kenaf, tree bark, sugar cane, chopped leaves and common garden mulch. The problem with cellulose products and some nut hulls is that they contain bacteria which, at downhole conditions, decompose the cellulose readily. Anyone who has been on the shale shakers when a slug of rotting paper and cottonseed hulls comes back to the surface is immediately aware of the decomposition problem by the horrendous odor from a nasty, soured mud system pouring out of the well.

Soon, the mud industry introduced other products including inert particulates, spun fibers and hydrophilic natural polymers. The particulates include various minerals: calcium carbonate flakes in various sizes, gypsum, and metallic flakes (aluminum, tin, zinc, copper and steel). Someone found that acid-soluble spun mineral fibers were very useful for trapping other particles in a web at the formation face. Good stuff, and it can be removed from the well unlike other materials.

The greatest improvement over the last several years has been with polymers. Some of these are simple monomer systems that create stiff, hard gels. Others are cross-linked polymer systems that create a gel with the viscosity of a rubber pencil eraser. Some of these are natural polymers, but the more recent ones are synthetics whose properties can be adjusted by modifying polymer length and end groups. The possibilities are endless.

Work was completed several years ago on a borehole strengthening process. Graphitic particles were pumped and displaced at near fracture pressure to create hoop stresses in the wellbore wall to resist fracturing. This system is used to increase leakoff-test-indicated fracture pressures that allow drilling in deeper hole sections without breaking down a weak shoe.

Graphitic particles are also used for lost circulation and pipe-to-wall friction reduction. It is slick in any mud system, because of its molecular structure. Mono-molecular “sheets” of graphite slide over one another when stressed, reducing torque and drag. Since graphite is pure carbon, it can be used on wells with bottomhole temperatures above 600°F, if someone finds a mud system that will hold together at that temperature.

New lost circulation products and systems are introduced on a regular basis. Research, development and testing of these are in progress all over the globe. Some of the old standby systems are still being used; however, the new ones may be much less expensive than the “tried and true” techniques.

So, call your friendly mud man and ask him if he has something new to solve your lost circulation problem. You’ll find that they aren’t really an abomination that everyone spits on – well, at least some of them are not (that’s a joke). Actually, most of these folks are very good at solving lost circulation problems, and many new systems are worthy of consideration.